Amplitude-adaptive vector quantization system

ABSTRACT

An amplitude-adaptive vector quantization system intended for efficient signal coding and decoding. A mean value separation circuit (2) separates the mean value from an input signal which has been divided into blocks, and a tree-search vector quantizer (6) implements tree-search inner product vector quantization so that index information is truncated depending on the detected vector amplitude, thereby reducing the volume of information for transmission.

BACKGROUND OF THE INVENTION

1. Field of the Art

This invention relates to a high-efficiency signal coding and decodingsystem and, particularly, to an amplitude-adaptive vector quantizationsystem.

2. Background Technology

FIGS. 3 and 4 are block diagrams showing the coding and decodingsections, respectively, of a vector quantization system which uses theinner product for the computation of distortion. In FIG. 3, indicated by1 is the input signal vector derived from a series of input signalsdivided into blocks in K-samples, 2 is a mean value separation circuit,11 is the separated mean value, 13 is the mean separated input vector, 3is a mean value DPCM (Differential PCM) quantizer, 29 is a vectorquantizer, 31 is the amplitude gain produced through coding by thevector quantizer, 7 is an amplitude-gain DPCM quantizer, and 33 is theoutput vector index produced from the code of the vector quantizer. InFIG. 4, indicated by 19 is a mean value DPCM decoder, 20 is anamplitude-gain DPCM decoder, 24 is the DPCM-decoded mean value, 36 isthe DPCM-decoded amplitude gain, 35 is a vector quantizing decoder, 38is the decoded mean value separation vector, 23 is a mean value addingcircuit, and 39 is the output signal vector.

Next, the operation wil be described. In the coding section of FIG. 3,the input signal vector 1 which is a series of input signals provided inthe form of blocks is received by the mean value separation circuit 2which separates the mean separated input vector 13 from the intra-vectormean value 11. The separated mean value 11 is subjected to predictivedifferential quantization (DPCM quantization) by the mean value DPCMquantizer 3, and information of the separated mean value 11 is reduced.The mean separated input vector 13 is subjected to inner productcomputation by the vector quantizer 29 with the output vector read outof a code book which is a set of output vectors prepared in advance in astatistical manner so that an output vector yielding a maximum innerproduct is selected. The vector quantizer 29 produces the output vectorindex 33 for the input mean separated input vector and also the innerproduct value as the amplitude gain 31 of the input mean separated inputvector 13.

Vector quantization is expressed in terms of input signal vectors 13S=S₁ S₂, . . . , S_(K), mean value μ, amplitude gain σ*, and outputvectors y_(i) =[y_(i1), y_(i2), . . . , y_(ik) ], as follows. ##EQU1##where σ=|S-μ.U| and |yi=1 are assumed.

Using the inner product, the vector distortion computation is reduced tothe form of a product-sum which is easier for execution with a DSP(digital signal processor), and the amplitude component can be obtainedat the same time. The amplitude gain 31 produced by the vector quantizer29 is subjected to DPCM quantization by the DPCM quantizer 7 in the sameway as for the mean value 11.

The DPCM-quantized mean value 12, amplitude gain 32 and output vectorindex 33 obtained as described above are coded and then transmitted orrecorded.

The decoding section of FIG. 4 implements decoding for the threecomponents namely the DPCM-quantized mean value 12, DPCM-quantizedamplitude gain 32 and output vector index 33 provided by the codingsection. The DPCM-quantized mean value 12 is DPCM-decoded by the DPCMdecoder 19, which yields the decoded mean value 24. Similarly, theDPCM-quantized amplitude gain 32 is DPCM-decoded by the amplitude gainDPCM decoder 20, which yields the decoded amplitude gain 36.

The vector quantization decoder 35 outputs the normalized output vector37 by decoded the output vector index 33, the amplitude reproductioncircuit 22 reproduces the amplitude of the vector as output vector 38from the decoded amplitude gain 36, the mean value adding circuit 23adds the decoded mean value 24 to the decoded mean separated outputvector 38, and the output signal vector 39 is reproduced.

In the conventional vector quantization system using the inner productfor the computation of distortion, the amplitude component is obtainedafter vector quantization, and therefore the vector quantization indexcannot be reduced by varying the quantization stages of thetree-searched code book by the amplitude component or switching codebooks. If the use of inner product computation is eliminated forimplementing the above-mentioned adaptive control by the amplitudecomponent, it becomes difficult for the DSP (digital signal processor)to reduce the duty of hardware.

DISCLOSURE OF THE INVENTION

The present invention is intended to solve the foregoing problems, andits object is to provide a vector quantization system which is capableof reducing the volume of information of the vector quantization indexby varying the quantization stage of a tree-searched code book dependingon the vector amplitude component and also is capable of reducing theduty of hardware by means of DSP, while using the vector inner productfor the computation of distortion.

The inventive vector quantization system adopts tree search for thevector quantizer, and is equipped with a calculator for the amplitudecomponent of the mean separated input vector and an amplitude vs.quantization stage correspondence table for determining the quantizationstage which meets the magnitude of the evaluated amplitude component.The tree-search vector quantizer is designed to produce the amplitudegain based on the result of quantization by the given quantizationstage, and it includes an amplitude gain DPCM quantizer forDPCM-quantizing the evaluated amplitude gain, a second amplitude vs.quantization stage table for selecting the number of the quantizationstage by producing indices depending on the DPCM-decoded amplitude gain,and a selector for selecting an index.

In the vector quantization system of this invention, the amplitudecomponent calculator evaluates the amplitude component of the vector,and the tree-search vector quantizer produces an amplitude gain based ona deep or shallow quantization stage, in accordance with the amplitudevs. quantization stage table, in response to the magnitude of theamplitude component. After DPCM quantization and DPCM decoding, theamplitude gain is entered to the second amplitude vs. quantization stagetable, which re-evaluates the quantization stage on the basis of thequantized amplitude gain and selects an output vector index depending onthe quantization stage. The decoding section, by having a secondamplitude vs. quantization stage correspondence table, determines thequantization stage from the DPCM-decoded amplitude gain, and the outputvector index can be decoded.

According to another vector quantization system of this invention, theoutput vector is searched by tree search of n's notation system, and theassignment of variable length (will be termed "code") to the outputvector is made to express the code of output vector at the node of treeselected during the search of output vector at the last stage, bytruncating the lowest position (will be termed "LSB") of the code ofoutput vector at the last stage of tree structure. In addition, thestage of tree structure of the vector to be outputted is determined fromthe amplitude gain derived from the amplitude gain of the last stageoutput vector with respect to the input vector through DPCM quantizationand DPCM decoding, the code of output vector obtained accordingly at thelast stage is truncated from LSB.

The inventive vector quantization system first implements vectorquantization up to the last stage by tree search of n's notation systemfor the mean value separation vector, and the amplitude gain resultingfrom the last quantization stage is assumed to be the amplitude gain ofthe output vector selected during the search up to the last stage. Theamplitude gain at the last stage is subjected to DPCMquantization/decoding, an output vector of a shallow stage of treestructure or deep stage of tree structure depending on a large or smallvalue of the resulting amplitude gain is selected through the truncatingoperation from LSB for the code of output vector obtained at the laststage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the coding section of the amplitude-adaptedvector quantization system according to an embodiment of this invention;

FIG. 2 is a block diagram of the decoding section;

FIG. 3 is a block diagram of the coding section of the conventionalvector quantization system;

FIG. 4 is a block diagram of the decoding section;

FIG. 5 is a block diagram of the tree-search vector quantizer;

FIG. 6 is a block diagram of the quantizer at each stage of thetree-search vector quantizer;

FIG. 7 shows the coding section of the vector quantization systemaccording to another embodiment of this invention;

FIG. 8 is a block diagram of the decoding section;

FIG. 9 is a block diagram of the tree-search vector quantizer;

FIG. 10 is a detailed block diagram of the vector quantizer using theinner product for the computation of distortion;

FIGS. 11a and 11b are diagrams explaining the relation between thetree-search and index code according to an embodiment of this invention;and

FIG. 12 is a diagram explaining the relation between the tree-search andindex code according to another embodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of this invention will be described with reference to thedrawings. FIGS. 1 and 2 are block diagrams of the coding section anddecoding section, respectively, of the vector quantization systemaccording to this invention. In FIG. 1, indicated by 1 is the inputsignal vector, 2 is a mean value separating circuit, 11 is the separatedmean value, 3 is a mean value DPCM quantizer, 12 is a DPCM-quantizedmean value, 13 is the input vector which has the mean value, 4 is anamplitude component calculation circuit for the input vector, 5 is afirst amplitude vs. quantization stage correspondence table fordetermining the stage of the tree-search vector quantizer from themagnitude of the calculated amplitude component, 6 is a tree-searchvector quantizer which uses the inner product of the mean separatedinput vector for the distortion computation in vector quantization andproduces the amplitude gain 14 with the stage number depending on theoutput of the first amplitude vs. quantization stage correspondencetable 5, 7 is an amplitude gain DPCM quantizer, 15 is the DPCM-quantizedamplitude gain, 16 is the DPCM-decoded amplitude gain, 8 is a secondamplitude vs. quantization stage correspondence table for determiningthe stage of quantization for the output vector in accordance with thevalue of DPCM-decoded amplitude gain, 9 is an index buffer which fetchesthe output vector index at each quantization stage of the tree-searchvector quantizer, 10 is an output vector index selector which selects avector index to be outputted from the index buffer 9 in response to thesignal 17 indicating the output vector stage number from the secondamplitude vs. quantization stage correspondence table 8, and 18 is theoutput vector index.

FIG. 2 shows the decoding section of the vector quantizer according tothis invention, in which indicated by 19 is a mean value DPCM decoder,24 is the DPCM-decoded mean value, 20 is an amplitude gain DPCM decoder,25 is the DPCM-decoded amplitude gain, 26 is the decoded output vector,27 is the amplitude-reproduced output vector, and 28 is the outputsignal vector.

FIG. 5 is a block diagram of the tree-search vector quantizer 6 shown inFIG. 1. Indicated by 40 are vector quantizers which use the innerproduct of vectors at stages for the distortion computation, 41 are codebooks including normalized output vectors at the stages, 42 are theoutput vector indices as the results of quantization at the stages, and43 are the amplitude gains as the result of quantization at the stages.

FIG. 6 is a block diagram showing the operation of the vector quantizerwhich uses the inner product at each stage for the distortioncomputation. Indicated by 44 is the output vector index as a result ofquantization by the vector quantizer at the preceding stage, 45 is anaddress counter which determines an output vector to be read out of thecode book in accordance with the index of the preceding stage andproduces the code book readout addresses 46 sequentially, 47 is theoutput vector which is subjected to vector computation with the inputvector read from code book 41 13, 48 are multipliers, 49 is an adder, 50is the inner product value signal for the input vector and outputvector, 51 is a maximum inner product detector which produces a strobe52 upon detecting the maximum inner product value, 53 is an amplitudegain latch which holds the input inner product value signal 50 as theamplitude gain only upon receiving the strobe 52 from the maximum innerproduct detector 51, and 54 is an index latch which holds the readoutaddress 46 of the entered code book only upon receiving the strobe 52from the maximum inner product detector.

Next, the operation will be described.

In the coding section of FIG. 1, the input signal vector 1 to be codedis separated into the mean value 11 and mean separated input vector 13by the mean value separation circuit 2. The mean value 11 is quantizedby the DPCM quantizer 3, and the mean separated input vector 13 issubjected to computation of the magnitude of amplitude component by theamplitude component calculating circuit 4. In order to avoid anintricate computation, the amplitude of absolute value may be used asshown in the following. For a mean-value separation vector X, inputsignal vector S and mean value μ, the amplitude σ is given as follows.##EQU2## The tree-search vector quantizer 6 implements tree-searchquantization for the mean separated input vector 13, delivers the outputvector index at each quantization stage to the index buffer 9, andprovides the amplitude gain 14 based on the quantization result ofquantizers with the quantization stage number which has been selected onthe basis of the magnitude of the amplitude component by the amplitudevs. quantization stage correspondence table 5.

The amplitude vs. search stage correspondence table is designed suchthat it provides a shallow quantization stage in response to a smallamplitude component and provides a deep search stage as the amplitudecomponent becomes larger.

The amplitude gain 14 evaluated by the tree-search vector quantizer 6,is quantized by the DPCM quantizer 7 to have the DPCM quantizedamplitude gain 15, and the DPCM-decoded amplitude gain 16 is entered tothe second amplitude vs. quantization stage correspondence table 8. Thesecond amplitude vs. quantization stage correspondence table 8 revisesthe quantization stage number of the output vector index 18 which isfinally outputted on the basis of the DPCM-decoded amplitude gain 16,and the index selector 10 makes selection from among the indices atquantization stages held in the index buffer 9 in accordance with thesignal 17 indicating the output vector stage number. Consequently, theDPCMquantized mean value 12, DPCM-quantized amplitude gain 15 and outputvector index 18 are obtained as a coding output.

The decoding section of FIG. 2 receives the mean value 12, amplitudegain 15 and output vector index 18 which have been DPCM-quantized by thecoding section. The mean value DPCM decoder 19 and amplitude gain DPCMdecoder 20 implement decoding for the mean value 24 and amplitude gain25. The amplitude vs. quantization stage correspondence table 8 producesfrom the amplitude gain 25 the quantization stage signal which indicatesthe same quantization stage number of the vector quantizer in the codingsection, and sends it to the vector quantization decoder 21. The vectorquantization decoder 21 decodes the output vector index 18 in accordancewith the quantization stage number provided by the second amplitude vs.quantization stage correspondence table 8, and the normalized outputvector 26 is obtained. The amplitude reproduction circuit 22 multipliesthe decoded amplitude value 25 and adds the mean value 24 which has beendecoded by the mean value adding circuit 23 to produce the output signalvector 28 as a decoded output.

In the tree-search vector quantizer of FIG. 5, each vector quantizer 40which uses the inner product for the distortion computation determinesan output vector providing a maximum inner product from among outputvectors selected in each code book 41 on the basis of the vectorquantization output vector index of the previous stage, sends the outputvector index 42 to the quantizer of the next stage, and outputs theoutput vector index 42 and the amplitude gain 43 which is then innerproduct calculation result, as a quantization result of that stage.

FIG. 6 shows in more detail the vector quantizer of each stage, in whichthe address counter 45, upon receiving the previous stage vectorquantizer output vector index 44, provides an output vector address tobe compared with the input vector 13 in accordance with thetree-structure output vector. When the output vector 47 is read out ofthe code book 41, it is subjected to product-sum with the input vector13. When the obtained inner product value 50 of vector has becomemaximum with respect to the input vector, the maximum inner productdetector 51 issues a strobe, by which the amplitude gain latch 53 holdsthe inner product value 50 and the index latch 54 holds the outputvector address 46. At the time point when the address counter 45 hasoutputted all output vector addresses 46 for the input vector 13, thevalues held in the amplitude gain latch and index latch are outputted asan amplitude gain 43 and output vector index 42 of that stage.

According to this embodiment, as described above, the inner productcalculation is used for the distortion computation section of the vectorquantizer, the quantization stage number of the tree-search vectorquantizer is controlled by the magnitude of the vector amplitudecomponent, and the final quantization stage number is determined fromthe quantization decoding result for the obtained amplitude gain,whereby the duty of the hardware can be reduced by using a DSP (digitalsignal processor) and it becomes possible to reduce the volume ofinformation of the vector quantization index.

Another embodiment of this invention will be described referring toFIGS. 7-10.

In the coding section of FIG. 7, indicated by 61 is the input vectorwhich is a series of input signals made into blocks of K-piece units, 62is a mean value separation circuit for the vector, 63 is the separatedmean value, 64 is the mean separated input vector, 65 is a vectorquantizer implementing the amplitude-adapted tree search, 66 is the codeof output vector index in the last stage quantization, 67 is theamplitude gain for the input vector of the output vector in the laststage quantization, 68 is a mean value DPCM quantizer, 69 is anamplitude gain DPCM quantizer, 71 is the DPCM-quantized mean value, 72is the DPCM-quantized amplitude gain, 73 is the DPCM-decoded amplitudegain, 70 is an index bit length control circuit which truncates from LSBthe code 66 of output vector index in the final quantization inaccordance with the DPCM-decoded amplitude gain 73, and 74 is the indexcode of the final output vector produced by truncation from LSB for thecode 66 of the last stage output vector index in accordance with theamplitude gain 73.

In the decoding section of FIG. 8, indicated by 75 is a mean value DPCMdecoder, 76 is an amplitude gain DPCM decoder, 77 is the DPCM-decodedmean value, 78 is the DPCM-decoded amplitude gain, 79 is a vectorquantization decoder which decodes the index code 74 of the outputvector which has been coded by the coding section using the DPCM-decodedamplitude gain 78, 80 is the decoded normalization output vector, 81 isan amplitude reproduction circuit, 82 is the amplitude-reproduced outputvector, 83 is a mean value adding circuit, and 84 is the output signalvector.

FIG. 9 is a block diagram of the tree-search vector quantizer, showingthe quantization operation by the quantizer 96 of each stage between theoutput vector read out of the code book 97 of each stage and the inputvector 64, while making reference to the output vector index 98 of theprevious-stage quantizer.

FIG. 10 is a detailed block diagram of the vector quantizer which usesthe inner product for the distortion computation. Indicated by 100 is anaddress counter which produces the readout address 103 sequentiallywhile mapping the output vector address in accordance with the outputvector index 98 of the previous-stage quantizer, 101 is a multiplier,102 is an adder, 104 is the inner product value of input/output vectors,105 is a maximum inner product detector which issues a strobe upondetecting a maximum inner product value, 106 is an index latch whichholds the code book address and outputs it as the index, and 107 is anamplitude gain latch which holds the inner product value as an amplitudegain.

Next, the operation will be described.

In the coding section of FIG. 7, the input signal vector 61 received asblocks derived from a series of input signals is subjected to separationof the intra vector mean value 63, and it provides the mean valueseparation vector 64. The separated mean value 63 is subjected to DPCMquantization by the mean value DPCM quantizer 68.

The mean separated input vector 64 is subjected to quantization by thevector quantizer 65 as follows. It is assumed that the output vector istree search of n's notation system (n is a power of 2). FIG. 11 showspart of a binary tree and quad tree, as example. The code of indexappended to the output vector of each node of tree structure isdetermined as shown in FIG. 11 for examples. The quad tree of FIG. 11(b)has its codes indicated in binary notation at the upper position and inquad notation at the lower position. As will be appreciated from thefigure, the code increases by log₂ n bits (one bit in n's notationsystem) ate each increment of the stage number, and nodes of the treestructure can be traced from lower toward upper stage of the treestructure by truncating log₂ n bits from LSB at a time.

By implementing tree search up to the last stage for the output vectorcode book, the mean-value separation input vector is rendered vectorialquantization using the inner product for the distortion computation. Thetree search vector quantizer is expressed as a connection of vectorquantizers as shown in FIG. 9, and the vector quantizer of the m-thstage is expressed by the block diagram of FIG. 10. In FIG. 10, uponreceiving the output vector index 98 of the vector quantizer of the(m-l)th stage, the address counter 100 produces the address of theoutput vector to be compared with the input vector 64 on the basis ofthe tree structure output vector of n's notation system. After theoutput vector has been read out of the code book 97, it is subjected tothe product-sum operation with the input vector 64. When the resultingvector inner product value 104 becomes maximum with respect to the inputvector, the maximum inner product detector 105 issues a strobe, causingthe amplitude gain latch 107 and index latch 106 to hold the innerproduct 104 and output vector address 103, respectively, and at the timepoint when the address counter 100 has outputted all output vectoraddresses 103 for the input vector 64, the value held in the index latchis outputted as an output vector index 108 of the m-th stage. If them-th stage is the last stage, the value of the amplitude gain latch 107is also outputted.

In FIG. 7, as a result of tree-search vector quantization up to the laststage for the mean value separation vector 64 entered in the tree-searchvector quantizer 65, the index code 66 of the last-stage output vectorand the amplitude gain 67 for the input vector 64 of the last-stageoutput vector are outputted, and the amplitude gain DPCM quantizer 69provides the DPCM-quantized amplitude gain 72 and amplitude gain 73which is the DPCM-decoded version of 72. The amplitude gain which is theinner product of vector indicates the amplitude component of inputvector, in case of implementation of proper vectorial quantization, andtherefore mapping is conducted for the tree search quantization stage onthe basis of the DPCM-decoded amplitude gain 73 so that an output vectorof a shallow stage of tree structure is selected for a small amplitudegain or an output vector of a deep stage is selected for a largeamplitude gain. As has been mentioned on FIG. 11, in order to obtain theindex code of the output vector selected in the search process, theindex bit length control circuit 70 implements the truncation operationfor the index code of the last-stage output vector from LSB by log₂ nbits at a time for the stage determined depending on the amplitude gain73. For the mapping of the DPCM-decoded amplitude gain 73 to the searchstage number, predetermined ones are stored in advance for both thecoding side and decoding side. In this way, the output vector in themidst of the tree structure is also selected in accordance with theamplitude gain, and a shorter bit length results in more efficientquantization.

In the decoding section of FIG. 8, the mean value and amplitude gainDPCM decoders 75 and 76 implement DPCM decoding, and the DPCM-decodedamplitude gain 78 is used by the vector quantization decoder 79 inmapping for determining which stage the output vector index code 74 islocated in the tree structure. the normalized output vector 80 decodedby the vector quantization decoder is subjected to amplitudereproduction by the amplitude reproducing circuit 81 and mean valueaddition by the mean value adding circuit 83, and the final outputsignal vector 84 is decoded.

Although shown in the foregoing embodiment is the method of truncatingfrom LSB in units of log₂ n bits for the last-stage index in the treestructure of n's notation system in correspondence to the DPCM-decodedamplitude gain, another possible method is that a tree structure of n'snotation system is assumed to be part of a binary tree structure and thecode is truncated in 1-bit units from LSB. In this case, the followingprecaution must be taken. In FIG. 12 showing a binary tree, collectionof nodes with underlined codes makes a quad tree. Namely, any treestructure of n's notation system (n is a power of 2) can be assumed tobe the result of pull-off of stages of a binary tree in accordance withthe rule. In the case of a quad tree, for example, although the outputvector exists only at nodes with underlined codes in FIG. 12 as a codebook, when a vector which is the average output vector of two underlinednodes in much lower stage is outputted for nodes without underline, itbecomes possible to truncate the code in one bit units. Consequently,the volume of information generation by the amplitude component can becontrolled more precisely.

According to the other embodiment of this invention, as described, theoutput vector code book of the vector quantizer is configured in thetree search structure of n's notation system, tree search vectorquantization is implemented up to the last stage by truncating the indexcode from the low-order bit in accordance with the rule and byoutputting the output vector of nodes in the midst of tree search, andthe tree-search quantization stage is determined from the decodedamplitude gain obtained by DPCM coding/decoding the amplitude gainobtained by quantization of the last stage thereby to obtain the indexcode of the above-mentioned node, whereby it becomes possible to reducethe volume of information of the index in a sense of adaptation to theamplitude component of vector without incurring the complexity of vectorquantizer used for the inner product calculation which allows thereduction of duty of the hardware by using a DSP or the like.

According to this invention, as described, index information istruncated in accordance with the detected vector amplitude component,thereby reducing the volume of information transmitted, whereby thehardware can be made smaller to reduce its duty through the use of DSPor the like.

What is claimed is:
 1. An amplitude-adaptive vector quantization systemcomprising:a mean value separating circuit which receives K-dimensionalvectors derived from a continuous series of input signals divided intoblocks of K-samples and separates out a mean value of said input signalvectors; a predictive differential quantizer which implements DPCMquantization for said separated mean value; an amplitude componentcalculation circuit which calculates the magnitude of amplitudecomponents of mean separated input vectors; a first amplitude vs.quantization stage correspondence table which determines a quantizationstate of tree-search vector quantization in correspondence to themagnitude of said calculated amplitude components; and a tree-searchvector quantizer which calculates the inner product between each of saidmean separated input vectors and normalized output vectors with zeromean arranged to have a tree structure, implements a tree-search for anoutput vector providing a maximum inner product sequentially up to alast stage, provides a maximum inner product at each stage as anamplitude gain at each stage along with an output vector index, andoutputs selectively an amplitude gain of a quantization stage prescribedby said first amplitude vs. quantization stage correspondence table; anindex buffer which holds the output vector index of each quantizationstage obtained during the quantization of said tree-search vectorquantizer; an amplitude gain DPCM quantizer which implements DPCMquantization for said output amplitude gain to provide a DPCM-quantizeddifferential amplitude gain and implements DPCM decoding to provide areproduction of DPCM-quantized amplitude gain; a second amplitude vs.quantization stage correspondence table which provides a truequantization stage selection signal from said DPCM-decoded reproductionamplitude gain; a coding section which has an index selector forselecting an output vector to be outputted finally from among outputvector indices of all stages held in said index buffer, and transmits orrecords three elements, including said DPCM-quantized mean value,DPCM-quantized amplitude gain and selected output vector, as a codingoutput; and a decoding section including a mean value DPCM decoder andamplitude gain DPCM decoder which implement DPCM decoding for the DPCMquantized mean value and amplitude gain coded by said coding sectionrespectively, an amplitude vs. quantization stage correspondence tablewhich has the same content as said second amplitude vs. quantizationstage correspondence table to determine from the DPCM-decoded amplitudegain a quantization output of which stage of coding section is theoutput vector index decoded by a vector quantization decoder, saidvector quantization decoder implementing decoding for said output vectorindex by switching output vectors adaptively in response to an output ofsaid amplitude vs. quantization stage correspondence table, an amplitudereproducing circuit which implements amplitude reproduction of saiddecoded normalized output vector on the basis of said DPCM-decodedamplitude gain, and a mean value adding circuit which adds saidDPCM-decoded mean value to said amplitude reproduced vector to producean output signal vector.
 2. An amplitude-adaptive vector quantizationsystem according to claim 1, wherein each of said amplitude vs.quantization stage correspondence tables in case of controlling thedepth of tree of tree-search vector quantization adaptively inaccordance with the amplitude gain, comprises an amplitude gain vs.quantization stage correspondence table which provides a quantizationstage number n which is a depth of tree search, in terms of a dimensionr of tree (n is an integer greater than one), amplitude gain σ, andconstants a and b, as n=a.log_(r) (σ/b).
 3. An amplitude-adapted vectorquantization system comprising:a mean value separating circuit whichreceives K-dimensional vectors derived from a continuous series of inputsignals divided into blocks in K-samples units (K is an integer greaterthan one) and separates out a mean value of said input signal vectors; amean value predictive differential (will be termed "DPCM") quantizerwhich implements DPCM quantization for said separated mean value; atree-search vector quantizer of n's notation system (n is a power of 2)which receives the mean separated vectors provided by said mean valueseparation circuit as input vectors, calculates the inner productbetween each of the mean value separation vectors and normalized outputvectors read out of a tree-search code book of n's notation system,implements tree-search for an output vector providing a maximum innerproduct sequentially up to a last-stage, and provides a then maximuminner product value as an amplitude gain along with an output vectorindex providing a maximum inner product at a last-stage quantization,and an amplitude gain DPCM quantizer which implements DPCM quantizationfor the last-stage quantization amplitude gain provided by said vectorquantizer to produce a DPCM-quantized amplitude gain through DPCMquantization, implements DPCM decoding, and produces a DPCM-decodedamplitude gain; a coding section including an index bit length controlcircuit which receives an index code of the last-stage quantizationoutput vector provided by said vector quantizer and truncates the indexcode for a specified number of bits from the lowest bit position (willbe termed "LSB") in correspondence to the magnitude of said decodedamplitude gain value, and tree-search code book of n's notation systemconstructed to express the index code of output vector at nodes of eachstage selected during the search up to the last stage through truncationin log₂ n bit units from LSB for the code of last-stage output vectorindex with the intention of use by said vector quantizer; and a decodingsection including a mean value DPCM decoder and amplitude gain DPCMdecoder for respectively decoding the mean-value and amplitude gaincoded by said coding section, a vector quantization decoder whichrecognizes the index code length of the output vector to be decoded fromthe amplitude gain decoded by said amplitude gain DPCM decoder, anamplitude reproduction circuit which multiplies the decoded amplitudegain to the decoded output vector and a mean value adding circuit whichadds the DPCM decoded mean value to the multiplication result.
 4. Anamplitude-adapted vector quantization system according to claim 3,wherein said tree search code book of n's notation system (n≧4), in caseone quantization stage has an index code length of 1 bit, is providedwith an output vector having for the index a code with a length of 1-jbits (j<log₂ n), said code book which including, for all or part of 1and j, an output vector that is a mean output vector, with itshigh-order (1-j) bits being coincident, among output vectors having1-bit code, said index bit length control circuit being provided with awidth for the control of truncating bits in accordance with theDPCM-decoded amplitude gain.